The last few decades have seen a proliferation of theoretical research into the evolutionary origins of cooperation and the dynamics of its spread. The literature has revealed several key insights into this enigma [1]}.
Population structure is perhaps the most widely discussed mechanism that can promote cooperation [2]}, [3]}, and it has been studied by computer simulation [4]}, [5]}, [6]}, mathematical analysis [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, [18]}, and experiments with human subjects [19]}.
In structured populations individuals interact only with their neighbors – through either physical or social ties – and behaviors also spread locally. Population structure has the potential to favor the evolution of cooperative behaviors that would otherwise be disfavored in a well-mixed populations [2]}, [3]}, [4]}, [8]}, [13]}, [14]}.
In network-structured populations, for example, nodes represent individuals and edges typically represent social interactions between connected individuals [7]}, [8]}, [9]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [6]};
in set-structured populations, each individual is located in one or more of several social circles [36]};
and in multilayer-structured populations, social interactions occur in multiple different domains, such as on-line and off-line interactions, and payoffs to an individual are summed across domains [37]}, [17]}.
